FIG. 1 depicts a diagram of the salient components of wireless telecommunications system 100 in the prior art. Wireless telecommunications system 100 comprises the depicted data-processing systems, which are interrelated as shown. The equipment that constitutes system 100 includes radio base station equipment, switching equipment, and service-control equipment, and is described in detail below. Wireless telecommunications system 100 provides wireless telecommunications service in well-known fashion to mobile station 101 when present in any of location areas 110-i, for i=1 through I, wherein I is a positive integer and equals two in the prior-art depiction. System 100 provides service in accordance with the Global System for Mobile Communications (GSM) set of standards and in well-known fashion.
Some of the equipment present in system 100 is associated with a specific location area 110-i. Such equipment includes base transceiver station 102-i, base station controller 103-i, mobile switching center 111-i, and visitor location register 112-i. In contrast, some other equipment present in system 100 is not associated with a specific location area. Such equipment includes home location register 113, gateway mobile switching center 114, and public-switched telephone network 120. Mobile station 101 is not fixed to a particular location area, but can be associated with any location area, depending on its present location. As depicted, mobile station 101 is presently associated with location area 110-1.
Base transceiver station (BTS) 102-i is mobile station 101's access point to system 100. The BTS performs speech encoding, encryption, and other functions. Communications between mobile station 101 and a BTS 102-i occur over an air interface. These communications, which include control signalling as well as voice and data traffic, are carried over designated channels via known methods. System 100 can include a different number of BTSs than that depicted.
Base station controller (BSC) 103-i controls one or more BTSs. For example, the BSC handles radio channel allocation, frequency administration, receives power and signal measurements originating from the mobile stations, and controls handovers between BTSs that are connected to the same BSC. As depicted in FIG. 1, BSC 103-i controls BTS 102-i. Although BSC 103-i is depicted as being geographically remote from BTS 102-i, it can be co-located with a BTS or with mobile switching center 111. System 100 can include multiple BSCs, even within a given location area 110-i. 
The interface between BSC 103-i and each BTS 102-i is known as the “Abis” link or interface. The communications carried over the Abis link include both voice/data information and control information. These communications are carried over a traffic channel and a signaling channel.
Mobile switching center (MSC) 111-i provides at least some of the switching capability across location area 110-i. MSC 111-i comprises a switch that orchestrates the provisioning and control of telecommunications service to the mobile stations. System 100 includes a plurality of MSCs, each responsible for a geographic region of the network, in this case a particular location area. MSC 111-i can be connected to multiple BSCs, and it can interface with other MSCs. The interface between the mobile switching center and each base station controller is called the “A” link or interface.
MSC 111-i also enables a subscriber of system 100 to place a call to, or receive a call from another network, such as Public Switched Telephone Network (PSTN) 120 or other wireless networks.
Gateway mobile switching center (GMSC) 114 serves as a gateway between two networks, in this case the wireless network portion of system 100 that serves the location areas, and PSTN 120.
Visitor location register (VLR) 112-i is equipment for storing a database of the subscribers who have roamed into the jurisdiction of MSC 111-i, which is served by VLR 112-i. The data stored in VLR 112-i is either received from HLR 113 or collected from mobile station 101, or both. VLRs 112-1 and 112-2 communicate with HLR 113 via “D” interfaces, communicate with each other via a “G” interface, and communicate with MSCs 111-1 and 111-2, respectively, via “B” interfaces. In some configurations, VLR 112-i is integrated with MSC 111-i. 
Home location register (HLR) 113 is equipment for storing a central database that contains details of each mobile station subscriber who is authorized to use one or more of location areas 110-1 and 110-2 as the subscriber's home network. HLR 113 stores details of every subscriber identity module (SIM) card issued by a wireless service provider. Each SIM has a unique identifier called an “international mobile subscriber identity,” or “IMSI.” An IMSI is a primary key to each HLR record. Another identifier that is associated with the SIM is a “MSISDN,” which is often referred to as a “mobile subscriber integrated services digital network-number.” Each MSISDN is also a primary key to the HLR record of a subscriber.
The MSISDN together with the IMSI are two important numbers used for identifying a mobile subscriber. The IMSI identifies the SIM, and the MSISDN is used for routing calls to the subscriber. The HLR data for a subscriber is stored for as long as a subscriber continues to subscribe to the service provided by the wireless service provider associated with HLR 113.
Some subscribers who are receiving wireless services within location area 110-1 or 110-2 are present within their home service area; information about these subscribers, and their SIMs and mobile stations, are stored within HLR 113. Other subscribers who are receiving wireless services within location area 110-1 or 110-2, however, are not present within their home service area. Instead, they are considered “roamers” within a service area that is operated by a service provider that is different than their home service provider. The subscriber data associated with a roamer are stored at an HLR other than HLR 113 and that is operated by the distant home service provider.
Regardless of whether a subscriber is in their home area or in a roaming area, the VLR that is currently serving the subscriber and the subscriber's own HLR continually exchange data with each other. This exchange of data is important, not only to provide communication services to the subscriber and associated mobile station, but also to keep track of and provide only those services that are allowed for each subscriber and mobile station. For example, a subscriber might be allowed to use one set of services in their home area, but a different set of services while roaming in a different area. The services allowed in the roaming area are typically a subset of the services allowed in the subscriber's home area.